The invention is related to a method for operating a valve train of an internal combustion engine, with the valve train having at least one main camshaft on which at least one cam carrier is provided in fixed rotative engagement and axially shiftable between at least two axial positions, and which is associated with an actuator to axially shift the cam carrier into a target position selected from the axial positions, wherein the cam carrier is associated with at least one shift gate that cooperates with the actuator to shift the cam carrier, wherein the actuator has a driver that is pushed out in the direction of at least one sliding slot of the shift gate for shifting the cam carrier, wherein the sliding slot has an ejection ramp in an ejection region for expelling the driver from the sliding slot until completion of the shift, and wherein a voltage induced in the actuator by the expulsion is detected. The invention further relates to a valve train of an internal combustion engine.
The valve trains on which this method is based are generally known. They are used for internal combustion engines in which the working cycle of the gas exchange valves of individual cylinders of the internal combustion engine can be influenced for improving thermodynamic capacity. The at least one cam carrier which can be also called a cam piece is arranged in fixed rotative engagement with and axially shiftable on the main camshaft. Multiple valve-actuating cams, i.e. at least two, are usually assigned to the cam carrier. Each of these valve-actuating cams has an eccentricity provided to operate one of the gas exchange valves of the internal combustion engine at a particular rotational angle position of the main camshaft. The valve-actuating cams thus rotate jointly with the main camshaft so that the respective gas exchange valve of the internal combustion engine is operated by the respectively associated valve-actuating cam or its eccentricity at least once per revolution of the main camshaft. The valve-actuating cam interacts hereby preferably with a roller cam follower of the gas exchange valves by making contact with the latter.
Preferably multiple valve-actuating cams are provided that can be assigned to different cam groups. The valve-actuating cams of a cam group only differ for example from each other with regard to the angular position of their eccentricity or the extension thereof in radial direction (height) and/or in circumferential direction (length). As a result of its axially shifting, the cam carrier can be shifted into at least two axial positions, for example a first and a second axial position. In the first axial position, the gas-exchange valve is operated by a first one of the valve-actuating cams, and in the second by a second one of the valve-actuating cams, which are assigned to the same cam group. In particular the opening time, the opening duration and/or the stroke of the gas exchange valve can thus be selected especially in dependence on an operating mode of the internal combustion engine by shifting the cam carrier. Of course, more than two valve-actuating cams can be provided per cam group, and a corresponding number of axial positions.
The shifting of the cam carrier in axial direction is implemented for example with the aid of a position adjuster that includes a shift gate on the cam carrier and an actuator having a fixed location, usually on a cylinder head of the internal combustion engine. The actuator has for example an extendible driver that can engage in particular a helical or spiral-shaped shift path or shifting slot of the shift gate. The shift path is provided on the shift gate that is assigned to the cam carrier. For example, the shift gate is provided on the cam carrier, or at least interacts with it for axially shifting. The shift path is preferably formed as a radial slot which extends through the circumference of the shift gate, i.e., forming an edged opening in it. The shift gate thus has at least one shift path in which the driver of the actuator can be inserted to shift the cam carrier. The current position of the cam carrier is referred to hereinafter as actual position, and the desired position as target position. The target position is selected from the possible axial positions of the cam carrier. Thereafter, the actuator is operated in such a way that the cam carrier is shifted towards the target position so that subsequent to the shifting, the actual position matches the target position.
The actuator is usually only constructed to push out the driver in the direction of the sliding slot. It does not have any means for moving the driver out of the sliding slot or to move it in again. Therefore, the sliding slot has the ejection ramp that is associated with the ejection region. The ejection ramp extends over the entire ejection region which substantially corresponds to a rotational angle range of the crankshaft of the internal combustion engine. The ejection ramp is arranged so as to ascend radially in the direction of rotation, i.e. the driver received in the sliding slot is brought completely out of the sliding slot by the end of the ejection ramp, or shifted to its initial position. To monitor whether the driver still remains in the sliding slot or has already been brought out of it by the ejection ramp, a voltage induced in the actuator during the expulsion is detected.
Basically, such an approach is known from DE 10 2004 030 779 A1, for example, the content of which is incorporated herewith by reference. Normally, a voltage difference between the induced voltage and an on-board supply voltage has to exceed a particular threshold level over a particular period of time. A confirmation signal or a feedback signal is generated only when this is the case. This indicates the successful expulsion of the driver from the sliding slot by the ejection ramp.